This invention relates to a birefringence type measuring device which is used for measuring physical values such as voltage, electric field or the like by utilizing a birefringence effect such as Pockels effect, and more particularly to a type thereof wherein a birefringence phase difference is detected for measuring voltages and the like.
A device for measuring voltage or the like utilizing Pockels effect has been widely known. In this device, a linearly polarized light entering into a birefringence substance is split into two beams, the polarization directions thereof being perpendicular with each other. In the substance, the beams transmit along different paths that exhibit different refractive indices, thereby causing a phase difference between the two beams. When an electric field corresponding to a voltage to be measured is applied across the birefringence substance, the refractive indices along the light paths change, thus varying the phase difference between the two beams.
The device utilizing the Pockels effect is advantageous in it is easy to insulate that it does not disturb the electric field to be measured, and it exhibits better frequency characteristics than those devices utilizing potential transformers and the like. However, since the known devices detect the variation of the phase difference by measuring the amplitude or the intensity of the light output, the measurement is liable to be disturbed by the loss in light intensity along the paths and the detected variation of the phase difference tends to be not proportional to the voltage or electric field to be measured. Furthermore, since the light intensity is converted into an electric signal by a photoelectric converter such as a photodiode, the amplitude of the signal tends to be varied by the drift of the characteristics of the converter due to temperature variation or the like.
The construction of a conventional voltage measuring device utilizing the Pockels effect, and the difficulties of the device will be described in more detail with reference to FIG. 1.
A laser light source 1 emits a linearly polarized laser light E.sub.1 of a single frequency .omega. with a plane (or direction) of polarization directed at an angle of 45.degree. relative to the x axis. The laser light transmits in the direction of the z axis through a substance 2 exhibiting the Pockels effect, a quarter wave length (.lambda./4) plate 4, and an optical analyzer 5 to a photodiode 6. Assuming an orthogonal coordinate system inclusive of the x and z axes, when a voltage V to be measured is applied from a source 3 to the substance 2 as shown in FIG. 1, a component e.sub.z of an electric field expressed as e.sub.z =V/L is created in the direction of the z axis as shown in FIG. 1, wherein L represents the length of the substance 2 measured along the z axis. Under the electric field e.sub.z, the substance 2 exhibits a birefringence phenomenon in the principal axes x and y, changing the refractive index n.sub.x for the x-component E.sub.x of the linearly polarized light E.sub.1 to n+.DELTA.n/2, and the refractive index n.sub.y for the y-component E.sub.y of the linearly polarized light E.sub.1 to n-.DELTA.n/2, wherein n represents the refractive index of the substance 2 at the time when the z-component e.sub.z of the electric field is zero and hence n.sub.x is equal to n.sub.y, and .DELTA.n represents an increment of the refractive index n. The increment .DELTA.n is proportional to the z-axis component e.sub.z of the voltage V applied to the substance 2 manifesting the Pockels effect, that is .DELTA.n=ke.sub.z wherein k is a constant.
As a consequence, when the linearly polarized light E.sub.1 having components E.sub.x and E.sub.y (hereinafter termed optical wave components) passes through the substance 2 of the length L, a phase difference .gamma. of the following magnitude appears between the two optical wave components E.sub.x and E.sub.y. ##EQU1## wherein .lambda. represents the wavelength in vacuum of the laser light.
Thus, by measuring the phase difference .gamma. between the two components E.sub.x and E.sub.y, the magnitude of the component e.sub.z or the voltage V can be determined from the equation (1).
Assuming that a represents the amplitude of the linearly polarized laser light E.sub.1 polarized in a plane at an angle of 45.degree. with respect to the x and y axes, the laser light having an angular frequency .omega. can be expressed as EQU E.sub.1 =ae.sup.j.omega.t
and the components E.sub.x and E.sub.y of the x and y axes are ##EQU2##
When the laser light E.sub.1 having the two components passes through the Pockels cell 2 with an electric field e.sub.z applied thereto, a phase difference .gamma. as follows is provided between the components E.sub.x ', E.sub.y ' and components E.sub.x, E.sub.y, where components E.sub.x ', E.sub.y ' are the x and y axes components delivered from the cell 2. ##EQU3## That is, the output of the Pockels cell 2 is polarized elliptically.
The output laser light of the cell 2 is then passed through the quarter wave length plate 4 principal axes of which extend in alignment with x and y axes so that the optical wave components thereof are changed to E.sub.x " and E.sub.y " which are expressed as follows. ##EQU4##
Therefore, the output of the quarter wave length plate 4 is also an elliptically polarized wave.
The optical analyzer 5 passes only one component E.sub.45 of the output of the quarter wavelength plate 4, which is polarized in a plane forming an angle of 45.degree. with respect to either of the x axis and y axis. ##EQU5## The component E.sub.45 is then applied to the photodiode 6 which converts the component E.sub.45 into a current signal I proportional to the intensity .vertline.E.sub.45 .vertline..sup.2 thereof. That is, the current signal I is expressed by ##EQU6##
From the equation (6), it is apparent that the output electric signal from the device shown in FIG. 1 is not proportional to .gamma. but is proportional to sin .gamma., and hence an additional device for calculating ##EQU7## is required. Furthermore, each time .gamma. varies by 360.degree., the output electric signal I resumes the original value, thus restricting the measuring range to .+-.180.degree..
Still another and more serious difficulty of the device shown in FIG. 1 is that a drift of characteristic inevitably occurs during the conversion step of the light intensity .vertline.E.sub.45 .vertline..sup.2 into the electric signal I carried out by the photodiode 6. More specifically, a dark current tending to be varied by temperature flows through the photodiode 6 in addition to the current signal I of equation (6), thus causing a serious error in the measurement. Furthermore, even in a case where the phase difference .gamma. is constant, the electric signal I is varied by the variation of the amplitude a of the laser light from the source 1, and also by light losses in light paths.